S-(2,3-Dichlorotriazinyl)glutathione (SDTG) was synthes-ized and shown to be an effective alkylating affinity label for recombinant maize glutathioneS-transferase I (GST I). InactivationofGST IbySDTGatpH followedbiphasic pseudo-first-order saturation kinetics. The biphasic kinetics can be described in terms of a fast initial phase of inactiva-tion followed by a slower phase, leading to 42 ± 3% residual activity.